A thin film type EL device (hereinunder referred to as "thin-film EL device") using a thin-film fluorescent layer has attracted attentions in place of a dispersion EL device using a powder of a zinc sulfide (ZnS) fluorescent material, because the former can provide a high luminance while the latter cannot provide a sufficient luminance so that the development thereof as a light source of illumination has been inevitably abandoned.
The thin-film EL device has a light emission layer composed of a transparent thin film and scarcely scatters the light entering from the outside and the light emitted in the interior of the light emission layer which would otherwise produce halation or blurring. Since the thin-film EL device produces a clear image having a high contrast, it has attracted attentions as a display for mounting on vehicles, for terminal devices and the like, and as a device for illumination.
For example, the fundamental structure of a thin-film EL device which uses manganese (Mn) as the luminescence center in ZnS is a double dielectric structure in which on a light-transmitting substrate 1, a light-transmitting electrode 2 consisting of a tin oxide (SnO.sub.2) layer or the like, a first dielectric layer 3, a light emission layer 4 consisting of a crystalline thin film having ZnS as a host material and Mn as the luminescence center impurity, namely, a ZnS:Mn thin film, a second dielectric layer 5, and a back electrode 6 consisting of an aluminum (Al) layer or the like are laminated in series in that order, as shown in FIG. 1.
The equivalent circuit of the thin-film EL device can be represented as three capacitors consisting of the first dielectric layer 3, the light emission layer 4 and the second dielectric layer 5 which are connected to each other in series, as shown in FIG. 2.
The process of the light emission of the thin-film EL device is as follows.
When a voltage is applied between the light-transmitting electrode and the back electrode, the electric field induced in the light emission layer attracts the electrons which have been trapped in the order of the interface and accelerates the electrons so as to provide a sufficient energy. These electrons collide with the orbital electrons of Mn which is the luminescence center and excite them. When the thus-excited luminescence center returns to the ground state, light is emitted.
In order to increase the voltage applied to the light emission layer in such a thin-film EL device, it is considered to be good that the relative dielectric constants .epsilon..sub.1 and .epsilon..sub.2 of the first and second dielectric layers are sufficiently larger than the relative dielectric constant .epsilon..sub.3 of the light emission layer (.epsilon..sub.l &lt;.epsilon..sub.r1, .epsilon..sub.r2). That is, since the electric capacitances of the first and second dielectric layers thereby become sufficiently larger than that C.sub.l of the light emission layer (C.sub.l &lt;C.sub.r1, C.sub.r2), almost all the voltage applied from the outside to the device is applied only to the light emission layer.
For the above-described reason, for the dielectric layers on both sides of the light emission layer, a material having a high dielectric constant, in other words having a relative dielectric constant .epsilon. of about 20 to 100 is used. In addition, in order to prevent a current from flowing on the thin-film EL device, a material having a resistivity as high as about 10.sup.13 to 10.sup.14 .OMEGA.cm is used.
However, the voltage-luminance characteristic curve of the thin-film EL device having such a structure is such as the curve b shown in FIG. 12, and unless the driving voltage is comparatively high, the desired luminance is not obtained.
The sealing structure of a conventional thin-film EL device is composed of a protective glass 8 which is pasted to the substrate 1 by an epoxy adhesive 7, and a silicon oil 9 which is charged into the space formed between the protective glass 8 and the surface of the thin-film EL device, as shown in FIG. 1.
A thin-film EL device having such a sealing structure, however has a poor air-tightness which sometimes allows water to mix with the oil. The water often breaks the thin-film EL device, which is a cause of lowering the reliability.